You use a semicolon after a statement. This is a statement:

var foo = function() {

  alert(bar);

};

because it is a variable assignment (i.e. creating and assigning an anonymous function to a variable).

The two things that spring to mind that aren't statements are function declarations:

function foo() {

  alert(bar);

}

and blocks:

{

  alert(foo);

}

Note: that same block construct without semi-colon also applies to for, do and while loops.